It is generally known that when a molten metal is poured into a mold, the surface of the metal becomes oxidized. The structure of a casting becomes of poor quality on account of the presence therein of oxide inclusions affecting the mechanical properties of the casting, its strength in particular. Moreover, in some cases, the castings are to be made of highly active metals which are liable to self-inflame when contacting the ambient air, which might cause a case of emergency. The use of flour of sulfur for prevention of self-inflammation of a metal results in polluting the casting with the components of the flux, which likewise affects the mechanical properties of the casting. To avoid surface oxidation of the metal, besides the application of flour of sulfur, there are also used inert gases -argon and others, or else there is a pre-evacuation of the mold. The last-mentioned technique, however, is applicable solely in pressure-die casting or else in arc-type vacuum casting machines.
There is known a pressure-die casting machine (cf. SU Inventor's Certificate No. 438,496; Int. Cl..sup.2 B 22 D 17/14) comprising half-molds movably mounted on a baseplate, a pouring bowl and a unit for pressing-in the metal.
To produce a casting, vacuum is generated in the working cavity defined by the half-molds. Then a portion of the metal, sufficient for making one casting, is poured into the pouring bowl, and the pressing-in unit is operated to inject the metal into the working cavity. The vacuum generated in the working cavity protects the metal against the formation of oxide films.
However, a machine of this kind is operable solely for casting machine parts of relatively small dimensions, whereas casting of large parts, involving the use of bulky cores, is virtually impossible.
There is further known a pressure-die casting machine (cf. JA Patent No. 52-26741; Int. Cl..sup.2 B 22 D 17/20) comprising a bed supporting a movable plate and a stationary one having the half-molds mounted thereon. The bed further supports a unit for pressing-in the metal, a mechanism for ejecting a casting and a device for purging the working cavity of the mold or die.
This machine is suitable for pressure-die casting. The working cavity is filled with the inert gas in a closed space, with the supply of the inert gas protecting the metal against oxidation. However, same as in the previously described case, the machine cannot be used for making castings of large parts.
There is still further known, a squeeze-out casting machine comprising a baseplate having reciprocably mounted thereon the half-molds and sealing jaws with pour-in ports adapted to overlap closingly as the half-molds are brought together, and a centering pin rigidly secured on the baseplate and supporting a frame with the core unit defining jointly with the baseplate, the sealing jaws and the portions of the half-molds, adjoining the baseplate, a metal-receiving chamber (cf. Vinogradov, V. N. "Casting Molds for Non-Ferrous Alloys", Album of Designs, MASHINOSTROYENIYE Publishers, Moscow, 1981, Sheet 28, in Russian).
The machine is suitable for making large castings but of a limited range of metals. At present, machines of this type are used solely with aluminum and zinc alloys. The absence of facilities for protection against the action of the ambient atmosphere upon the melt results in the formation of oxide films in and on the castings, with their mechanical properties becoming correspondingly affected. The present-day requirements of reducing the weight of castings and enhancing their mechanical strength call for the use of more lightweight alloys. However, alloys of lightweight metals require positive protection against the action of the ambient atmosphere.